The present invention relates to a process for the distillation of a mixture composed of at least 2,2′-diisocyanatodiphenylmethane (2,2′-MDI), 2,4′-diisocyanatodiphenyl-methane (2,4′-MDI) and 4,4′-diisocyanatodiphenylmethane (4,4′-MDI), in order to recover 2,4′-diisocyanatodiphenylmethane containing very little 2,2′-diisocyanato-diphenylmethane as well as mixtures of 4,4′- and 2,4′-diisocyanatodiphenylmethane containing very little 2,2′-diisocyanatodiphenylmethane.
Diisocyanatodiphenylmethane isomers are constituents of polyisocyanate mixtures of the diphenylmethane series which are produced during the phosgenation of aniline/formaldehyde condensates, also referred to herein as polyaminopolyphenyl-polymethanes.
The condensation of aniline and formaldehyde and the phosgenation of polyaminopolyphenylpolymethanes are sufficiently well disclosed in the prior art.
After the phosgenation of polyaminopolyphenylpolymethanes, phosgene is removed completely. Then the higher homologues of diisocyanatodiphenylmethane (also called polyisocyanatopolyphenylpolymethanes) are separated out. From the remaining mixture of isomeric diisocyanatodiphenylmethanes, mainly comprising 2,2′-diisocyanatodiphenylmethane, 2,4′-diisocyanatodiphenylmethane and 4,4′-diisocyanato-diphenylmethane (for simplification purposes also referred to herein as MDI-isomer mixtures or MDI crude mixtures), pure diisocyanatodiphenylmethane isomers or mixtures of two or three isomers can then be isolated, depending on the product specification required. A variety of isolation processes, based on a distillation or crystallization procedure or a combination of distillation and crystallization, are disclosed in the prior art.
For example, WO 02/070581 describes a process for preparing 2,4′-diisocyanato-diphenylmethane by isolation from a MDI crude mixture.
Furthermore, DE 2 631 168 A discloses a process for preparing diisocyanatodiphenyl-methane isomers, using a distillation process, by isolation from a polyisocyanate mixture obtained by the phosgenation of aniline/formaldehyde condensates. The useful materials 4,4′-diisocyanatodiphenylmethane and 2,4′-diisocyanatodiphenylmethane are obtained in this way. The isolation of 2,2′-diisocyanatodiphenylmethane is not described.
For many applications, the presence of 2,2′-diisocyanatodiphenylmethane in the useful materials or mixtures of useful materials is not desirable. However, during the simple distillation of crude diisocyanatodiphenylmethane mixtures, the isolation of 2,2′-MDI is impossible, or possible only in a technically difficult and economically costly manner.